Multiple terminals, such as ISDN station sets , are connectable to a single digital subscriber line with an Integrated Services Digital Network (ISDN) multipoint interface, defined in Recommendation I.430 of the International Telegraph and Telephone Consultative Committee (CCITT). The terminals connected to a digital subscriber line share the B-channels and D-channel that comprise the line. The D-channel is used for the exchange of control information between the terminals and the network. This control information is required to set up calls, redirect calls, and terminate calls. The B-channels are used for circuit switched voice and data calls and serve different terminals at different times. For a terminal to use a B-channel, a B-channel must be allocated to the terminal. The switching system allocates a B-channel to a terminal by establishing connections in the switching system to allow access to the B-channel and defining the B-channel as being used only by that terminal.
In prior ISDN arrangements having a single terminal connected to a digital subscriber line, a B-channel on the digital subscriber line is allocated to the terminal when an incoming call arrives for the terminal. In a multi-point arrangement, if a channel is allocated to a terminal whenever a call arrives for that terminal and the digital subscriber line only includes two B-channels, as is the case for the ISDN basic rate interface, at most two terminals on the digital subscriber line could be rung in response to calls at any one time. More than two terminals connected to a single digital subscriber line may share a directory number. However, if the switch must allocate a B-channel to a terminal before ringing a terminal, at most two terminals on the digital subscriber line could be rung in response to any call.
Consider an incoming call to a directory number shared by four terminals on a digital subscriber line that has two B-channels. Although only one terminal will actually answer the call to the shared directory number, it is desirable to have as many of the terminals as possible ring in response to the call to allow the terminal users flexibility in determining who will answer the call. If a B-channel must be allocated for a terminal before the switch requests the terminal to ring in response to an incoming call, at most two of the four terminals could ring in response to a call to the directory number the four terminals share. Thus, a problem exists in efficiently offering calls to terminals sharing a digital subscriber line that has a limited number of B-channels.
Race conditions arise when processing calls for multiple terminals sharing a digital subscriber line having a limited number of B-channels. For example, if nothing is done to insure that a B-channel remains available for a terminal to answer a call, the call may become unanswerable. A call becomes unanswerable when a B-channel cannot be allocated to any of the terminals addressed by the call. Checking that a B-channel could be allocated to a terminal addressed by a call at the time the call is offered does not insure that the call can be answered. Even though a B-channel could have been allocated when the call was initially offered, the B-channel may be allocated to another terminal before a terminal addressed by the incoming call attempts to answer.
This race condition creates confusion for both the calling and called parties and uses switching system and network resources inefficiently. Having a call become unanswerable provides ambiguous information to the calling party about the status of the called party. At first, the calling party hears ringing. After the call is determined to be unanswerable, the calling party would either continue to hear ringing or would receive a congestion or busy tone indicating that the call had become blocked. If the calling party continued to hear ringing, he could conclude that the called party was away from his desk or otherwise unavailable to answer the call. Alternatively, first hearing ringing and then hearing a busy or congestion tone could be confusing to the calling party since he is not accustomed to such a response.
The called party is also provided with confusing information. First, the called party's terminal rings in response to the call indicating to him that he may answer the call. However, when he attempts to answer the call, he finds that the call cannot be answered. Furthermore, he is not assured that anyone else will be allowed to answer the call. Consider an example in which an executive is expecting an important call. Her terminal begins to ring and she notes from the calling directory number displayed in a terminal display that this is the call she was expecting. However, on attempting to answer the call, she discovers that the call cannot be answered since no B-channel can be allocated. The executive has been disturbed unnecessarily to answer a call that cannot be answered. Furthermore, she now is aware of a call and does not know if anyone else who shares the directory number will be able to answer the call.
In addition, offering a call and then later determining that the call is unanswerable results in an inefficient use of resources. Imagine that a call is placed from a caller in Australia to someone in the United States. When the call is offered to the called party, a number of inter-swtich and intra-switch resources are allocated for the call and remain allocated until the call is terminated. For example, if the calling party just lets the phone ring, the resources remain allocated. These resources have been tied up unnecessarily if the call subsequently becomes unanswerable.
Thus, recognized problems in the art are inefficiencies that occur in processing calls for multiple terminals sharing a single digital subscriber line and ambiguous information that is provided to the calling and called parties for these calls.